Prof Garel Rhys, chair of the Welsh Automotive Forum and one of the country's leading motor industry economists, helped introduce the company to Welsh government officials.

But while he supports the idea, he told the programme of his concerns about the jobs forecast.

"It increases expectation to an incredible height and where are those [jobs] going to come from?" he said.

"You need incredible multipliers to get to 6,000 - so I've thought when it was said that it was silly."

"If you could have 1,000-plus (jobs) on the Heads of the Valleys, fantastic - but a figure of 6,000 really doesn't do any good to anybody."

Deprived area

In response, Heads of the Valleys Development Company chief executive Michael Carrick said: "That is disappointing. I think we will create many more jobs than that."

The development promises to transform one of Wales' most deprived communities.

Alongside the racing circuit, there are plans for hotels, a grandstand, a technology park and a solar park to be created.

But those projects all rely on the racetrack going ahead.

A crucial part of the development involves securing the rights to host MotoGP - the motorbike racing equivalent of Formula 1 - which attracts global audiences of up to 300 million.

The company has said it aims to build the circuit in time to host the prestigious event in September 2015, but Week In Week Out has discovered that no deal has yet been signed with owners of the MotoGP races, Dorna Sports.

There are also delays in starting building work on the site because of planning issues.

The company hoped to have had diggers on the site last December.

But because the circuit is being built on 332 hectares (820 acres) of common land, the company has to de-register it, find an equivalent amount of land elsewhere and re-register that as common land.

The Planning Inspectorate for Wales said the process can take almost a year to complete.

For the developers, Mr Carrick said he remained confident the company can deliver but refused for reasons of commercial confidentiality to name any of the investors who he said are willing to buy into the scheme.

Funding questions

He added that a proposed £30m investment from Welsh government "is not a huge amount of public money" and called it "quite a modest contribution".

The Welsh government refused to say how much public money it is considering investing in the project.

But a letter seen by the BBC Wales programme shows the figure is £30m.

The programme also discovered the company is seeking a £20m commitment from the UK government to underwrite the project.

Conservative AM Antoinette Sandbach will later ask for an investigation into whether more public money should be spent on the development.

She also wants more transparency from the Welsh government over the deal, after being refused access to a report into the decision to award the development a £2m grant in 2012.

She asked the Auditor General for Wales Huw Vaughan Thomas to investigate the basis on which that grant was made.

He said the due diligence report into the grant had not considered concerns from other circuit owners who warned that few circuits make a profit - and that even if they attract big events, they can still lose millions of pounds because of the costs involved in hosting them.

Ms Sandbach said: "It doesn't look in sufficient depth of detail whether the claims the company make are accurate, deliverable - and actually the Welsh government and the Auditor General have identified that it is high risk.

"And if it is, then you need to assess how great is that risk, because otherwise we are going to lose that - and it's money that could be spent in other ways in Wales."

Business Minister Edwina Hart declined to be interviewed for the programme. Officials said they could not discuss future public funding for reasons of commercial confidentiality.

Blaenau Gwent council is backing the scheme and said it is satisfied with the company's business plan.

Council leader Hedley McCarthy told the programme: "A lot of those questions need to be put to developers because they are the only ones that can answer them."